Optical discs, such as CD-ROM (Compact Disc-Read Only Memory) or DVD-ROM (Digital Versatile Disc) media, have data stored as a series of lower "pits" formed within a plane of higher "lands". The pits are arranged in a spiral track originating at the disc's center hub and ending at the disc's outer rim. The CD-ROM and DVD-ROM formats allow for discs that are inexpensive to produce yet have high storage densities. DVD-5, which was launched in 1997, has a diameter of 120 mm and a capacity of 4.7 gigabytes (GB). However, such discs are pre-recorded and cannot be recorded on by the end user.
Rewritable optical discs typically come in the form of magneto-optical or phase-change discs. DVD-RAM (random-access-memory) are rewritable phase-change discs. These discs have a spiral tracking groove comprised of pits having a depth of about .lambda./4 n, or about 103 nm for a laser wavelength, .lambda., of 650 nm, and a refractive index, n, of 1.58. Data may be recorded by the end user in the grooves.
In phase-change discs, the recording layer is a material that can exist in two different solid phases. The material can be switched from one stable phase to another by appropriate heating by a read/write laser. Typical phase-change materials are multicomponent alloys that have a stable, compatible, crystalline phase and a metastable amorphous phase with different optical properties, namely, a different coefficient of reflectivity, R. Recording is accomplished by locally melting the recording material and then cooling it quickly enough to quench it in the amorphous phase. The material in the amorphous state (i.e., the recorded bit) can be erased by annealing by heating the bit to a temperature for a long enough period of time to recrystallize the bit. Typical phase-change materials include GeTeSb and AgInTeSb.
The specification for DVD-RAM as of August 1997 calls for a capacity of 2.7 GB. DVD-RAM discs contain no pre-recorded data (other than servo and/or format information). If any such data were recorded prior to sale to the end user, such data would detract from the 2.7 GB of recordable storage available to the end user.